The present invention relates to a variable resistance circuit suitably adapted for time constant changing of, e.g., a time constant circuit in a noise reduction system, frequency compensation equalizer, etc.
In a noise reduction system of audio equipment, a time constant of a low-pass filter is often switched so as to change characteristics of level expansion operation or level compression operation. A time constant circuit of such a low-pass filter is generally formed of a resistor and capacitor. In this case, one element of these resistor and capacitor is fixed while the other is selected from plural ones.
When a plurality of resistors or capacitors are selectively switched, the number of terminals used for the switching is inevitably increased, thereby increasing the manufacturing cost of products. In particular, when the time constant circuit is circuit-integrated in an IC package, unfavorable increase of cost due to increase in number of the external pins of IC is unavoidable. For instance, in FIG. 1, when the value of a capacitor 10 is fixed and resistors 11 and 12 are selectively used with capacitor 10, the following disadvantage is invited. Thus, in addition to output terminal (external pin) 13, independent input terminals (external pins) 14 and 15 numbering the same as that the resistors used are required, resulting in high cost in IC fabrications.